The United States gas transmission pipeline industry depends significantly on reciprocating compressors to achieve its gas pumping capacity. The industry currently operates over 4000 of these compressors. In general, such compressors include one or more cylinders within which reciprocating pistons are disposed. The pistons are driven by a traditional crankshaft that, in turn, is driven by an electric motor, diesel or natural gas burning engine, or other appropriate motor. A suction valve through which gas is drawn on the down or suction stroke of the piston and a discharge valve through which gas is expelled on the up or discharge stroke of the piston are disposed at the head of each cylinder. With both valves closed, the piston, cylinder walls, and cylinder head trap a volume of gas subsequently referred to as the compression volume. During the compression stroke, motion of the piston towards the head increases pressure of the trapped gas. During the re-expansion stroke, motion of the piston away from the head decreases pressure of the trapped gas. Piston rings encircle the pistons and bear against the cylinder walls to form a seal between the pistons and the cylinder walls during compressor operation. Most compressors are designed to employ double-acting pistons with a trapped compression volume of gas on either side of the piston. Each trapped volume, sometimes termed a "end", has its own suction and discharge valves. Such compressors function to pump and convey natural and other gases along a pipeline.
There are compelling reasons to operate reciprocating compressors at maximum efficiently. Efficient operation reduces the energy required to operate the compressors, which usually is derived directly or indirectly from fossil fuels. Efficient operation also minimizes emissions from leaky compressor components and directly benefits compressor operators by reducing operating costs. Compressor operators thus strive to minimize conditions that lead to reduced compressor efficiency.
Primary causes of reduced compressor operating efficiency are leaks that can develop as a result of day-to-day operation. Such leaks can occur between the piston rings and the cylinder walls, permitting gas to leak past the piston as it reciprocates within the cylinder. Leaks can also occur in the suction and discharge valves of compressors. A leaky suction valve allows some gas to migrate back through the suction valve during the compression stroke, discharge, and re-expansion strokes of the piston. Similarly, a leaky discharge valve can allow the migration of some gas back into the cylinder through the discharge valve during the re-expansion, compression or suction stroke of the piston. Either of these conditions results in reduced flow rate and thus reduced efficiency of the compressor. Such leaks also tend to worsen with time resulting in steadily deteriorating mechanical health of the compressor.
Obviously, early detection, diagnosis, and repair of leaky compressor components is crucial to maintain such compressors at peak operating efficiency. To accomplish this, many gas transmission companies have instituted maintenance programs guided by regular periodic testing and inspection of their reciprocating compressors. These inspections usually include periodic measurement of cylinder pressure as a function of changing cylinder volume as the piston reciprocates to produce a pressure versus volume diagram known as a PV card. From visual inspection of the measured PV card, a skilled and experienced operator can often diagnose the existence of cylinder problems such as leaky valves or leaky piston rings. Ultrasonic measurements can also be an effective supplement for detecting valve leaks in compressors.
While manual analysis of measured PV cards provide some ability to diagnose and pinpoint leaks in reciprocating compressors, it nevertheless exhibits numerous inherent problems and shortcomings. For instance, the accuracy and dependability of such analysis depends strongly on the experience and intuition of the individual analyzing the PV card. Accordingly, the diagnosis is subject to human mistakes and errors in judgment that render the results somewhat unreliable. This is important because misdiagnosed leaks can lead to expensive down time for repairs that may be unnecessary. In addition, manual analysis of the PV card provides only an indication of the general nature of a leak in the compressor. It usually does not provide a detailed analysis of the severity of the leak, does not express the problem in usable terms such as cost of wasted fuel, and cannot predict with any accuracy the overall performance of the compressor system with the leak and with the leak repaired.
Accordingly there exists a continuing and heretofore unaddressed need for a method of detecting and specifying compressor cylinder leaks that is reliable, not dependent upon human experience and intuition, that specifies detected leaks in detail including the location and severity of the leak, than provides usable information regarding the impact of the leak on compressor efficiency, and that infers the overall performance of the compressor system with and without the leak. It is to the provision of such a method that the present invention is primarily directed.